We investigated the effects of low-intensity resistance training on muscle size and strength in old men and women. Thirty five participants (59-76 years old) were randomly assigned into two groups and performed low-intensity (50% of one-repetition maximum) knee extension and flexion exercises with either slow movement and tonic force generation (LST; 3-s eccentric, 3-s concentric, and 1-s isometric actions with no rest between each repetition) or normal speed (LN; 1-s concentric and 1-s eccentric actions with 1-s rest between each repetition) twice a week for 12 weeks (2-week preparation and 10-week intervention). The LST significantly increased thigh muscle thickness as well as isometric knee extension and flexion strengths. The LN significantly improved the strength, but its hypertrophic effect was limited. These results indicate that even for older individuals, the LST can be an effective method for gaining muscle mass and strength.